The present invention relates generally to voltage transfer switches, and more particularly, to AC voltage source transfer methods for switch systems having a transformer downstream of the transfer switch.
Voltage transfer switches are commonly used to switch between a primary and one or more alternate power sources in the event of failure or instability of the primary source. Such transfer switches are commonly used in applications that require continuity of power, such as in hospitals and critical processes in both industrial and commercial settings. For example, in a power system having a primary voltage source and one alternate voltage source, fist and second switches are associated with the primary and alternate voltage sources, respectively. The switches are activated by a controller, such that upon a failure of the primary source, the first switch is opened to remove the primary voltage source from a load and the second switch is subsequently closed to connect the alternate source to the load, hence maintaining power to the load.
The “static switches” used for this switching function typically employ silicon controlled rectifier (SCR) devices. The controller applies signals to the SCRs' gate terminals to gate them into conduction and to commutate them off as necessary based on the condition of the primary and alternate voltage sources. Generally, the second switch is turned on as soon as possible after the load is disconnected from the primary source in an attempt to minimize the voltage disruption at the load side. In some situations, however, the timing of turning off the first switch and turning on the second switch is optimized based on the make-up of the system. For example, in systems having a transformer connected downstream of the switches, switching may be optimized to prevent the transformer from saturating due to the dc flux built up during the transfer event. Such transformer saturations are highly undesirable since they can cause large saturation currents to flow, which in turn can cause system failure due to source overloading or upstream protective breakers tripping.
In such optimized system, the additional delay that results from waiting for the optimum point to transfer may cause increased output voltage waveform disturbance and load current discontinuity, which are not acceptable to some critical loads. Additionally, when SCRs are used in such a voltage transfer switch optimized for minimum downstream transformer saturation currents, there are cases where the SCRs will not naturally commutate off for a significant period of time. This may prevent the controller from turning on the appropriate alternate source SCRs at the optimum time without creating a cross conduction situation, thus extending the transfer time.
The present application addresses these shortcomings associated with the prior art.